Ecological Functions

Zostera beds in general provide a wide array of ecological functions important for maintaining healthy estuarine and coastal ecosystems, creating essential habitat, and forming a basis of primary production that supports ecologically and economically important fisheries (Duarte et al., in press). Zostera marina provides habitat for numerous commercially important fish and shellfish species (Orth and Heck, 1980; Thayer et al., 1984; Orth and van Montfrans, 1987; Heck et al., 1989, 1995, 2003; Fredette et al., 1990; Short etal., 1993a; Dean and Haldorson, 2000; Beck et al., 2001; Short et al., 2001). In New South Wales, Australia, areas vegetated with Z. capricorni were found to have the greatest diversity and highest abundance of fish in shallow, marine regions, especially during the recruitment period (West and King, 1996). These included juveniles of many commercially important species such as the yellowfin bream (Acanthopagrus australis) and sea mullet (Mugil cephalus). Worthington et al. (1995) associated the great temporal and spatial variability in juvenile species in Z. capricorni habitats to variability in recruitment as well as subsequent mortality. Connolly (1994a,b) reports the significantly greater abundance of fish in Z. capricorni (reported as Z. muelleri) beds compared to unvegetated habitats and suggests that the use of these Zostera habitats may be related to both food availability and habitat structure (see Gillanders, Chapter 21). Similarly, Woods and Schiel (1997) found that the crab Macrophthalmus hirtipes used only Z. capricorni (reported as Z. no-vazelandica) beds, especially along patch edges of the seagrass, to the exclusion of other habitats as they fed on both the seagrass and sediment in the patches. Here, their burrows affected seagrass bed structure as crab burrow abundance at patch edges accelerated patch erosion. The relationships between the Zostera habitats and organisms using these habitats are very complex. In mixed beds of intertidal Z. capricorni (reported as Z. muelleri) and subtidal Z. tasmanica (reported as Heterozostera tasmanica), for example, differences in fish assemblage have been related more to the depth zone of the habitat than to the pres ence or absence of vegetation (Jenkins et al., 1997). Similarly, abundance of some gastropods was found to be significantly different between seagrass species in mixed beds of Z. caulescens and Z. marina (Toy-ohara et al., 1999).

Overall, the generally recognized value of Zostera beds as important nursery areas for many species has not been well quantified for most regions of the world (Heck et al., 2003). Heck et al. (2003) suggest that the structural component provided by seagrass beds may be the most important attribute contributing to their nursery function. There are, however, many recent studies continuing to confirm the greater habitat functions and secondary production of Zostera beds compared to adjacent unvegetated areas (Fredette et al., 1990; Costa, 1994; Orth et al., 1996; Dean and Haldorson, 2000). Yet the impacts of the continuing loss of these of these important habitats to fisheries is still not well quantified (Beck et al., 2001).

Some species associated with Z. marina habitat are in decline, such as flounder, cod and scallops. In Puget Sound, Washington juvenile salmonids are associated with Z. marina habitat and are listed as threatened ( 1salmon/salmesa/). The Z. marina limpet, Lottia alveus, became extinct after the 1930s wasting disease (Carlton et al., 1991). The brant goose (Branta bernicla), a species dependent on Z. marina as a primary food source, was abundant before the 1930s and has only partially recovered. Ducks, swans, and other species of goose use Z. marina as food and are known to stop in Z. marina areas during migration. The Pacific black brant migration is linked closely to the distribution of Z. marina at sites from Baja California, Mexico to Izembek Lagoon, Alaska (USA) (Wyllie-Echeverria and Ackerman, 2003). Zostera beds are also important foraging habitats for wading birds such as the heron (Matsunaga, 2000). The abundance of Zostera and waterfowl populations have been reported to co-vary in many areas. In British Columbia, Canada, expansion of Z. japonica habitat was associated with increases in dabbling ducks and brant (Baldwin and Lovvorn, 1994a) and waterfowl distribution within shallow water areas has been related to the presence of Zostera vegetation (Baldwin and Lovvorn, 1994a; Clausen et al., 1998). Conversely, the impacts of waterfowl on Zostera have been quantified and birds have been found to consume from a low percentage to more than half of Zostera biomass in areas studied

(Nienhuis, 1978; Baldwin and Lovvorn, 1994b; Portig et al., 1994; Schutten et al., 1994; Fox, 1996; Vermaat and Verhagen, 1996).

The importance of Z. marina to estuarine and coastal productivity was highlighted in the 1930s, when a large-scale die-off of Z. marina occurred on both sides of the Atlantic due to wasting disease (Rasmussen, 1977). The disease resulted in the loss of over 90% of the North Atlantic Z. marina population, and this loss had a catastrophic effect on estuar-ine productivity including the disappearance of the scallop (Argopecten irradians) fishery and drastic reduction in brant geese (Branta bernicla) populations (Milne and Milne, 1951).

In addition to functions as nursery and feeding ground, Zostera beds enhance the local productivity of an area through increased epibenthic and benthic production, providing substrate for epiphytes and enhancing benthic invertebrate production (Fredette et al., 1990). Zostera beds act as a filter of estuarine water, trapping and binding sediments (Rasmussen, 1977; Fonsecaetal., 1983; Fonseca and Fisher, 1986; Fonseca, 1992; Heiss et al., 2000) and dampening wave and current energy (Grizzle et al., 1996; Koch and Verduin, 2001). These plants also take up and bind contaminants (Lyngby andBrix, 1982; Francois etal., 1989; Ward, 1989; Hovenetal., 1999). Zostera beds produce and release O2 to the water, create and export organic material, and facilitate the accumulation of organic matter in the sediments (Costanza et al., 1997). Zostera is important in the nutrient cycling ofthe coastal ocean, increasing decomposition in the sediments, accelerating nutrient regeneration, and regulating nutrient cycles (Short, 1987; Hansen et al., 2000).

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